1. Field of the Invention
The present invention relates to an image processing method, and more particularly to an image processing method for determining an exposure state of a film image, such as underexposure, overexposure, etc., so as to process the image based on the exposure state.
2. Description of the Related Art
FIG. 11 is a graph showing the characteristics curve of negative film. A, B, and C indicate an exposure area of standard exposure, underexposure, and overexposure, respectively. As shown in the drawing, the negative image, which is underexposed or overexposed in photographing, is different from a standardly-exposed negative image in their gradation characteristics and gamma characteristics. Japanese Patent Application Laid open 4-107083 discloses a signal processing circuit, which uses a variable gamma so as to deal with image signals of various gradation characteristics. The signal processing circuit is an analog processing circuit, and has a variable gain amplifier circuit in a front step and a back step in a gamma correction circuit having a knee characteristics. The gain of these variable gain amplifier circuits is adjusted so as to change the gamma.
A density area of the overexposed and underexposed negative images is narrower than that of the standardly exposed image. Japanese Patent Application Laid-open No. 6-274946 discloses an image processing method which adjusts the tone of the soft-tone image and the hard-tone image.
When the gamma correction, etc. of the image signal is performed without logarithmic conversion, the underexposed or overexposed negative image is different from the standardly-exposed negative image in its gamma characteristics. Therefore, the gamma correction should be performed depending on the exposure state. If the overexposed or underexposed negative image is gamma-corrected in the same way as the standardly-exposed negative image, the underexposed image is dark and the overexposed image is too bright and the tone is not favorable.
Moreover, if the tone of the underexposed or overexposed image is adjusted in the same way as the standardly-exposed negative image when the tone of the soft-tone image and the hard-tone image is adjusted, there is a problem in that the tone changes too much and therefore the image is unnatural.
On the other hand, there is an image processing method for adjusting a white balance and a black balance of the R, G and B image signals which are obtained by picking up a color image. In this image processing method, a reference maximum value and the reference minimum value are determined from the R, G and B image signals. An offset and a white balance gain are determined so that the reference maximum value and the reference minimum value are predetermined gradations. The offset and the gain control of the R, G and B image signals are performed by the offset and the white balance gain. A histogram is obtained with regard to each gradation of the image signal, so that the reference maximum value and the reference minimum value can be determined from the histogram.
In the color image which has an extremely-high chroma area, the reference maximum value and the reference minimum value tend to be uneven. For example, as shown in FIG. 12, the reference minimum value of B is sometimes much larger than the reference minimum value of the other color.
In the above-mentioned case, if the offset and the gain control of the R, G and B image signals are performed using the reference maximum value and the reference minimum value obtained from the R, G and B image signals, there is a problem in that the white balance and the black balance are lost and the color is not reproduced satisfactorily.
That is, in the scene as shown in FIG. 12, the white balance gain of the B image signal is much stronger than that of the other color signals, so there is a problem in that the whole scene is blue (color failure).